I don't know how things are now but some clubs used to be more traditional that others...

Other that that, I'd say be sure that the stuff is original (see below) and that you could actualy imagine another person wanting to sing it.

I used to cringe when some people announced they were going to do their own song. I had a pretty good idea what it was coming - I've heard it 100s of times over with slight variation. It's nearly always sang in "tortured style", reflects their own woes inculing adolesence, girlfriends, etc. and it sends everyone else in the audience to sleep, that's if they haven't either gone to the bar or come in ready prepared with a razor blade.

If you can avoid that, I think all well and good, and good luck to you.